When it is attempted to improve productivity by increasing the total fineness of fiber bundles for the purpose of reducing the production cost for carbon fibers, there occur many problems to be solved in view of practical use or in view of production technologies. Thus, there have been occasions in which cost reduction could not be achieved sufficiently.
In order to solve these problems, Patent Document 1 proposes a technology in which scorching at the time of a stabilization treatment is suppressed by using a carbon fiber precursor fiber bundle having a high degree of roundness and a large value of single fiber fineness, and a carbon fiber bundle containing few interlaced single fibers and having excellent spreadability and excellent productivity in spite of its large total fineness, is obtained.
Furthermore, Patent Document 2 suggests a polymer which does not require a stabilizing process. Also, Patent Documents 3 and 4 propose a technology for enhancing oxygen permeability of a carbon fiber precursor fiber to thereby control the oxygen concentration distribution to be uniform within a stabilized fiber, and enhancing the tensile strength and tensile modulus of a carbon fiber thus obtained, by using a monomer having a bulky side chain as a copolymerized component of a copolymer.
Furthermore, Patent Document 5 proposes a technology for subjecting a polyacrylonitrile (PAN)-based carbon fiber precursor fiber to stabilizing while allowing heated air to penetrate into a yarn bundle on a mesh-shaped roller, and thereby suppressing heat accumulation inside the yarn bundle.
On the other hand, in view of the reduction of the production cost for carbon fibers, process stabilization is also a critical technology. For example, gelling of a spinning dope in a spinning process directly leads to process trouble, and there is a demand for an increase in thermal stability. Patent Document 6 discloses that a dramatic enhancement in thermal stability is achieved when a spinning dope is maintained at a high temperature of about 80° C., by esterifying methacrylic acid which is a component for accelerating a stabilizing reaction of a polymer.